Pro-lifers fall into all these spectrums as well. Assuming pro-lifers only believe that personhood starts at conception is patently false. Many pro-lifers believe personhood starts at the beginning of neural activity, even though they accept life as beginning at conception. Timber1972 takes the position that it is a person from the moment fertilization begins. Just like all pro-choicers don't advocate the extreme version of this debate, pro-lifers don't just occupy the opposite extreme. I think most people, pro-lifers and pro-choice, fall into the same belief (70% if I remember correctly) that the fetus shouldn't be aborted after the first tri-mester. The two groups have more in common than you think if you care enough to talk and listen to the other side.
For myself, I believe that personhood comes down to purpose and intent. On the left leaning side of the argument, the purpose is assigned by the parent. For instance; a woman gets pregnant and decides she doesn't want the baby, because of accident, rape or some other reason. She then aborts the baby, and the law condones it, because she has that choice. Now, the same woman decides to keep the baby, for her own reasons and purposes. Now the law considers that baby a person and is obligated to protect it. Nothing changed from the baby's perspective other than the mother making a decision to keep it. That decision warranted the right to life on that new person.
Same woman walking home after making her decision to keep the baby, encounters a mugger who beats her and takes her money...the baby dies in the process. The law now (in most states) has two charges for the mugger, assault for the mother and manslaughter for the baby...all because she bestowed personhood on it by choosing to keep it. So, the baby's rights and personhood hinge on purpose and intent, not from itself, but from the mother.
For those on the right of this issue, purpose and intent, are also at play. The difference being that the purpose and intent are coming from both the parent(s) and another being, God. Everything has a purpose, no matter how small or large, and if God has ordained a purpose for this fertilized egg, e.g. the right to grow into a baby and eventually an adult, then, as this side proposes, the embryo deserves the same protections as any other human.
That's an oversimplified view of the argument, I know, but it's hard to put all the scenarios into a meme post.